The physics in GT4 are much better, especially when it comes to everything about ascending, cresting and descending hills. The hills are more dramatic (and real,) and road surfaces now are uneven.
As far as the cars, especially the classics, yes, I'm also disappointed in some of the overlooked cars. Elite would have been nice, as would have TRs, Healey 100, MGB, MGA, MG TC, Jag D-Type/XKSS, AM DB4, Fiat Spider 124/2000, '59 Corvette, '77 Trans Am, BMW 507, Bentley Speed Six ...seeing as they had licensing done with these manufacturers anyway.